In the operation of various types of instruments, it is often desired that the instrument be set or maintained at a specific position relative to some reference datum (e.g., an axis, surface, plane, point, etc.). The success or optimization of the instrument's operation may depend on the ability to set the instrument to a precise position and/or maintain that position during operation. The nature of the instrument may be such that although the instrument has been correctly set at the required position, the instrument may deviate from that set position due to some influence such temperature variation, vibration, external impact, removal and reinstallation of a component, etc.
One example of a position-dependent instrument is a sample holder (or rotor) and its associated probe spinning module and probe RF circuit as utilized in nuclear magnetic resonance (NMR) spectrometry. As appreciated by persons skilled in the art, a typical NMR spectrometer typically includes radiofrequency (RF) transmitting/receiving electronics, a NMR sample probe, and a source of a strong, static magnetic field (B0 field) in which the sample probe is immersed such as a superconducting magnet. The NMR sample probe includes the probe spinning module, the sample holder (which contains a liquid or solid sample), and one or more RF coils that serve as the electromagnetic coupling between the RF electronics and the sample. The RF electronics are operated to irradiate the sample with RF energy (B1 field) and receive RF signals emitted from the sample in response to the RF input. The response signals are utilized to extract information regarding the sample. To greatly improve the resolution of spectral data for solid-phase samples and certain types of inhomogeneous liquid-phase samples, the sample holder can be supported in and rotated by the probe spinning module, with the sample holder functioning as a rotor and the probe spinning module functioning as the corresponding stator configured to spin the sample holder at a high rate (e.g., 106 RPM). In magic-angle spinning (MAS) techniques, the sample holder or rotor is positioned to spin at exactly the “magic angle” of 54.7° relative to the direction of the externally applied static magnetic B0 field to further enhance resolution.
NMR instrumentation in general requires a high level of precision and stability, and this is particularly the case in spinning techniques where the sample holder must be set and maintained at the magic angle throughout the experiment. To set the sample holder to the magic angle, the probe spinning module may be connected to an adjustment mechanism that enables the user to move or pivot the probe spinning module to the proper position. An adequate technique for precisely and easily setting the angular position of the probe spinning module and verifying that the as-set position is correct does not presently exist. Consequently, users of NMR spectrometers often run their experiments without realizing that the sample holder is not operating at the desired angle and thus without realizing that their acquired data has been compromised by the off-angle operation. Moreover, even assuming that the probe spinning module (and thus the sample holder) has been set up at the correct angle, various influences arising during an experiment may cause the sample holder to deviate from this angle. In particular, changing the temperature of the sample may change the dimensions of components or structures of the instrumentation that are inherently temperature sensitive (i.e., have coefficients of thermal expansion). Depending on where these components are located or how they function, an alteration in dimension might affect the angular position of the probe. Hence, when such components or structures are subjected to heat transfer, the probe spinning module may go off-angle. An acceptable solution for addressing this problem does not presently exist. To verify the proper positioning of the probe spinning module during an experiment, a user would need to pause the experiment, remove the probe spinning module containing the experimental sample, install a sample holder containing a reference sample designed to check the angle, operate the probe spinning module with the reference sample and make any necessary adjustments to the position of the probe spinning module, remove the angle-sensing sample holder and reinstall the experimental sample holder, and resume the experiment. For an experiment that requires sample irradiation at different predetermined temperatures, these steps would need to be undertaken several times throughout the course of the experiment. Moreover, the physical switching between probes in and of itself may adverse affect the proper setting of the angle of the probe.
Accordingly, there is an acknowledged need for improvements in the sensing of the position of instruments in general, and in particular for instruments associated with the spinning of NMR sample holders.